Typically, flexible printed circuit boards, which are configured such that a circuit pattern is formed on a thin insulating film, find applications in many fields including those of mobile electronic instruments, as well as automatic instruments and displays that must be flexible for their operation.
Particularly, there has been a strong demand for flexible printed circuit boards for mobile terminals such as smartphones. For example, flexible printed circuit boards are mainly utilized in NFC (Near Field Communication) antennas or digitizers of mobile terminals.
When installed in a display panel of an electronic instrument, such as a cellular phone, PDA, laptop computer, etc., a digitizer functions to recognize and represent the coordinates of touch points on the display panel, allowing for the natural representation of written letters or notes on the display panel.
Accordingly, the size of digitizers corresponds to that of display panels. With the tendency of display panels toward enlargement, such as in smartphones, tablet PCs, outdoor advertising displays, etc., digitizers have gradually increased in size.
With reference to FIG. 1, the demand for an electronic blackboard 1 is increasing in companies or educational institutions such as schools or academies because screen output is possible and writing is possible on the screen thereof, and the digitizer is applied in the electronic blackboard 1 so as to enable smooth and accurate writing.
The electronic blackboard 1 is installed indoors or outdoors and is thus used for lectures, seminars, conferences, presentations, etc., and includes a large display panel so that a large number of people can clearly see the screen.
Typically, a flexible printed circuit board is manufactured by etching copper foil that has been laminated on a flexible substrate or by printing a circuit pattern with a conductive paste or conductive ink on a flexible insulating film and then plating the circuit pattern.
However, such flexible printed circuit boards have to undergo etching or plating, and thus suffer from complicated manufacturing processes and high manufacturing costs.
Also, a coverlay is utilized to protect the circuit pattern, but incurs high manufacturing costs and makes it difficult to form to a thickness equal to or less than a predetermined level.
Also, the adhesion between the circuit pattern and the insulating film may decrease in the course of plating the terminal portion of the circuit pattern, undesirably leading to the circuit pattern easily separating from the insulating film.
Also, when the flexible printed circuit board is applied to flexible displays, the circuit pattern may be damaged and deformed due to repeated bending and warping, undesirably deteriorating operational reliability.
Particularly in the case where the digitizer is used for the electronic blackboard having a large screen, the size of the substrate has to be large so as to correspond to the large screen, undesirably causing problems including high manufacturing costs in the formation of the circuit pattern, easy separation of the circuit pattern from the insulating film, and damage and deformation of the circuit pattern due to the bending and warpage.
Also, the flexible printed circuit board is manufactured to have a multilayer structure so as to effectively dispose the circuits for its operation. For this, insulating films having different circuit pattern layers are attached using a bonding sheet.
Such a multilayered flexible printed circuit board includes via holes for electrically connecting the individual circuit pattern layers, but is problematic in that the manufacturing processes are complicated and the individual insulating films are adhered using a bonding sheet to be integrally formed, thus increasing the manufacturing costs.
Also, the multilayered flexible printed circuit board is problematic in that stable operational reliability is not maintained when the bondability of the bonding sheet deteriorates, and the thickness thereof is difficult to decrease, which undesirably increases the thickness of products including the same.